For example, an exhaust purification apparatus with an SCR catalyst (selective reduction type NOx catalyst) has been known as one in which an auxiliary agent is injected into exhaust gas and supplied to an exhaust purification device. As is well known, the SCR catalyst needs NH3 (ammonia) for reducing NOx contained in exhaust gas. In the exhaust purification system, a urea aqueous solution is injected as a reducing agent acting as an auxiliary agent from an injection nozzle disposed upstream of the SCR catalyst interposed in the exhaust passage. The NH3 produced by the urea aqueous solution being hydrolyzed by exhaust heat and water vapor contained in exhaust gas is used to accomplish a NOx reducing action of the SCR catalyst.
The NOx reducing action in the SCR catalyst is greatly influenced by the supply condition of the urea aqueous solution. In other words, in order to accomplish a good reducing action, it is necessary to supply NH3 to each section of the SCR catalyst as evenly as possible by fully dispersing and atomizing the urea aqueous solution in the exhaust gas, and preventing the urea aqueous solution from adhering to the wall surfaces of the exhaust passage and the like. To fulfill such a need, various measures have been suggested, which place an exhaust-gas agitating device in an exhaust passage. Such measures are disclosed, for example, in Unexamined Japanese Patent Publication No. 2006-29233 (hereinafter, referred to as Patent Document 1).
In an exhaust purification apparatus disclosed in Patent Document 1, as shown in FIGS. 1 and 2, a fin device with four fins, which functions as agitating means, is set upstream of an injection nozzle of an exhaust passage. When exhaust gas passes through the fin device, there generates a swirl flow by the action of the fins. This accelerates a urea aqueous solution to disperse into the exhaust gas.
In order to create a strong swirl flow that is proper to accelerate the dispersion of the urea aqueous solution by using the fin device, it is required to secure a decently wide fin area and set a fin angle large so that the exhaust flow direction may be changed at a steep angle. However, passage area at a position of the fin device is narrowed along with an increase of the fin area. At the same time, the increase of the fin angle changes the exhaust flow direction at a steep angle, leading to an increase in pressure loss. This results in an increase in exhaust pressure of the engine, which causes a deterioration in operative performance. In addition, a portion of the injected urea aqueous solution easily adheres to the inner circumferential surface of the exhaust passage due to the centrifugal force of the swirl flow. Such a phenomenon, too, becomes a factor in hindrance to the dispersion and atomization of the urea aqueous solution.
In this respect, there is still room for improvement in the exhaust purification apparatus disclosed in Patent Document 1 because there is a trade-off relationship between the acceleration of dispersion and atomization of the urea aqueous solution and the inhibition of an increase in engine exhaust pressure, and it is impossible to achieve both of these two at a high level.